This is a first renewal application of an excellent, highly productive team of a Bioengineer (G. Ateshian) with a Biochemist (W. Valhmu) who are studying the mechanisms of articular cartilage lubrication and/or failure of lubrication on loading/frictional characteristics of bovine articular cartilage. Based on previous theories and experiments, the authors propose new experiments to further characterize the relationships between friction and fluid pressurization of articular cartilage. The overall theory being tested is that pressurization shifts load away from contacting cartilage surfaces, thus decreasing friction until pressurization subsides. Three relevant hypotheses are to be tested in this renewal: 1/that friction depends on interstitial fluid support load, equilibrium frictional coefficient and solid-to-solid contact; that cartilage-on-cartilage contact fraction area is smaller than cartilage on glass. 2/ under normal loading, cartilage always maintains high interstitial fluid support with low friction and 3/ enzymatic degradation will defeat interstitial load mechanisms and increase the coefficient of friction. Using a unique load-or-displacement controlled frictional testing apparatus containing a microchip pressure transducer to measure cartilage interstitial fluid pressurization directly, the authors will systematically test these hypotheses through a series of carefully thought out permutations and combinations of load-versus-displacement controlled tests. The ability to test the fluid pressures within cartilage samples under these various, well-controlled, physiologically relevant loading conditions, in order to test various theories of cartilage lubrication and friction, is extremely important and unique. The investigators provide extensive evidence that this proposal is both logical and feasible. Their background review on the topic, along with their papers on the topic over the past few years are all outstanding. They provide both strong theoretical and experimental evidence to support the concepts that they are testing. The main concept is that fluid pressurization is critical to controlling cartilage friction. They will test that theory using cartilage-on-glass tests along with some very important and unique cartilage-on-cartilage tests at stresses from 0.05Mpa-5 Mpa, frequencies of 0.0005Hz - 0.5 Hz and up to 10 percent compressive strains, with and without enzymatic (collagenase, chondroitinase ABC and hyaluronidase) treatments.